This non-provisional application claims priority under 35 U.S.C. §119(a) on Korean Patent Application No. 2004-56828 filed on Jul. 21, 2004, which is herein expressly incorporated by reference.
1. Field of the Invention
The embodiments of the present invention relate to a novel black matrix, a method for preparing the same, a flat display device comprising the same, and an electromagnetic interference filter comprising the same. More particularly, the embodiments of the present invention relate to a black matrix comprised of a substrate, a photosensitive film, a Ni-plated layer and a Cu-plated layer, which is improved in blackening tone and electric conductivity, a method for preparing the same, and the applications of the black matrix, including a flat display device and an electromagnetic interference filter.
2. Description of Prior Art
A black matrix, which is the generic description of a light shielding film that separates the pixels of a screen's color filter, is applied to all display devices to prevent color mixing among R (red), G (green) and B (blue) and to improve the contrast therebetween. In liquid crystal displays, a black matrix is incorporated within a color filter by being positioned between any two of R, G and B picture elements in light emitting devices, such as fluorescent displays and OLEDs. In the case of plasma displays, a black matrix is prepared by additionally blackening copper wires of an electromagnetic interference filter, so that visibility can be prevented.
Thus far, most black matrixes have been prepared by patterning a film of metal such as chromium in a photolithography process, as representatively disclosed in U.S. Pat. Nos. 5,378,274 (Yokoyama et al.), 5,587,818 (S. Lee et al.) and 5,592,317 (Fujikawa et al.). Chromium is vacuum deposited on the internal surface of a display screen to form a light absorption coating, which is then patterned using photolithography, so as to prepare a black matrix. However, this conventional method has the disadvantage of having a high production cost. Further, the black matrix prepared by this conventional method exhibits high reflectance and a color filter incorporating this black matrix therewith is poor in visibility due to its high light reflectance.
To solve this problem, a method is suggested in which a metal chromium film is positioned between oxide layers, such as chromium oxide layers, so as to achieve low reflectance properties. The black matrix prepared according to this method has advantages of exhibiting high visible light absorption, being thin and having a high optical density. However, this method is environmentally unfavorable not only because chromium is converted to harmful chromium (VI), but also because heavy metal, including an oxidizing agent, such as ammonium-cerium (IV), is required.
Therefore, there has been a strong need for an organic black matrix paint that is inexpensive compared to chromic materials, does not produce pollution in the environment, and can be easily patterned, in the industry.
In the past decade, extensive research into organic black matrix materials has been conducted. In U.S. Pat. Nos. 4,822,718 (Latham et al.) and 5,176,971 (Shimamura et al.) dyed black matrix compositions comprising polyimide precursor binders are disclosed. These compositions, however, suffer from the drawbacks of short storage stability, low optical density after deposition, poor thermal stability, and low fade resistance. In addition, “dye-based” compositions produce dyed leachates while subsequent processes are conducted. In principle, dyes cannot make photo images, so no patterns may be formed without an additional photoresist layer.
In order to achieve higher optical densities, better thermal stability, and greater resistance to chemicals and fading, various pigment-dispersed (discriminated from “dye-based”) black matrix coating systems have been developed. However, none of the pigment-dispersed coating compositions developed thus far is found to satisfy a black matrix system in terms of high electric resistance, small film thickness, and high optical density.
Japanese Pat. Laid-Open Publication No. 8-34923 (Sekisui Chemical Industries, Ltd.) discloses a two-step process for preparing a black matrix, in which a composition comprising a photosensitive polymer and a black dye is applied onto a substrate and a black-dyed coating thus formed is exposed through a mask to form a desired pattern which is then colored with black dye. This method is too troublesome to be commercially applied.
When a matrix is prepared from an organic material, there occur problems in that light sensitivity is deteriorated upon frequent light exposure and the storage stability needs to be extended. Compositions having improved storage stability are disclosed in U.S. Pat. Nos. 5,626,796 (Tsujimura et al.), 5,639,579 (Hayashi et al.), 5,714,286 (Uchikawa et al.) and 5,866,298, and Japanese Pat. Laid-Open Publication Nos. 2000-147240, 11-143056, and 11-326606. However, none of the compositions having improved storage stability disclosed in the above literature can exhibit the high optical density of chromium black matrices. The reason is that since a resinous black matrix has poor absorbance compared to a metal black matrix, it must be made thick so as to obtain a necessary optical density. When a thick film is employed, a step problem is caused in subsequent processes, deteriorating the overall driving properties of the display device.
Japanese Pat. Laid-Open Publication No. 11-352310 discloses a method for preparing a black matrix using Ni and NiOx. Friendly as it is to the environment, this method needs a vacuum apparatus for forming Ni films, unlike a conventional sputtering apparatus, and in addition, it is difficult to maintain constant etching conditions.